For instrumentation of microporosity in cementitous materials, carbon nanotubes based capacitive ultrasonic transducers (cMUT) are promising sensors. Their interest lies in the combination of high working frequencies (1 GHz) with small dimensions (1 µm²). In the proposed device, the cMUT membrane is made of aligned single-walled carbon nanotubes (SWNT) bridging a gap over a command electrode. We will describe the realization process of the vibrating membrane and its characterizations. First step of the device realization is the dispersion of SWNTs in N-methylpyrrolidone. Then, nanotubes are aligned by dielectrophoresis (DEP) between metallic electrodes onto a SiO2 substrate. A metallic layer is deposited over the electrodes edges to prevent nanotubes from slipping when suspended. The underlying SiO2 is then etched to release the membrane. Relevant features of the membrane are nanotubes alignment and density. Via SEM imaging, we have linked them with DEP operating parameters, in agreement with theoretical properties of DEP. To put a figure on membrane features, we are working on SEM image processing for nanotubes recognition. The method is based on advanced noise removal and contrast enhancement. First results of identification and measurement of intermeshed nanotubes on SEM pictures will be presented. We also mapped the Young's modulus of a suspended membrane using an AFM in contact mode, over surfaces of about 1 µm² surface. It opens the way for calculation of localized Young modulus, Poisson's ratio and thickness measurement of the membrane. We will check for correlations between mechanical data and quantitative properties of the deposition obtained from image processing. The optimization of membrane realization process and characterization techniques are presented, describing the present progress of our cMUT project. Next step will be actuation of the membrane to demonstrate vibrations at low frequency.
